JP 2007-064268 A describes an exemplary automatic transmission mounted on a vehicle. In this automatic transmission, an engine rotation-driven oil pump supplies hydraulic pressure to a hydraulic control circuit. A hydraulic valve and a manual valve are provided in the hydraulic control circuit to switch a hydraulic circuit and to control a hydraulic pressure. Here, the hydraulic pressure is applied to friction engagement elements (e.g., a brake or a clutch) of a transmission mechanism. As a result, gear shifting, or range shifting, is performed.
This kind of automatic transmission includes a hydraulic control system, in which when foreign substances (such as iron particles or aluminum particles) accumulate within a hydraulic control valve (e.g., a solenoid or such), the operability of the hydraulic control valve may degrade. In the worst case, there is a chance that the hydraulic control valve may lock up due to jamming by the foreign substances.
For this reason, when the vehicle is stopped, a cleaning control is performed to cause the hydraulic control valve to perform a cleaning operation (for example, an operation where a spool valve repeatedly reciprocates). As a result, foreign substances are expelled from the hydraulic control valve, and accumulation of foreign substances within the hydraulic control valve may be prevented.